If I add smb://IP_ADDRESS/movies as a network share via samba then the movie posters and fanart don't show up. If I map the samba share as a drive letter and add this then the fanart and posters do show. So I guess XBMC wants to treat remote shares differently for optimization reasons maybe?

Is there a way I can force XBMC to use the posters and fanart from the samba share?